To meet the demand for wireless data traffic having increased since deployment of 4th generation (4G) communication systems, efforts have been made to develop an improved 5th generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post Long Term Evolution (LTE) System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 28 GHz or 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like.
In the 5G system, Hybrid frequency shift keying (FSK) and quadrature amplitude modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
There is an ever-increasing desire to improve the rate and volume of data throughput in communications networks, given the increase in data-hungry mobile applications and services. To achieve significant throughput enhancement in a practical manner, it is usually necessary to deploy a large number of cells in a given area and to manage them intelligently. As such, it is envisaged that future Fifth Generation (5G) deployment may take the form of an overlaid network in conjunction with existing Fourth Generation (4G) macro cells. The 5G small cells can be coupled with the overlaid 4G macro cells. In such a deployment, the 4G macro base stations will deliver control information to the small cells and user Equipment (UE), while data transmission will largely be performed between the small cell and the UE, usually operating at a different frequency.
In macro-cell only deployment, for the purpose of initial access, a UE performs periodical measurement of the macro base stations, and identifies the cells with, for example, the largest reference signal received power or reference signal received quality (RSRP/RSRQ) measured at the reference signals transmitted from the base stations, followed by cell selection for connection or handover purposes.
In a heterogeneous network deployment where macro and small cells operate on different frequencies, in order to identify opportunities to offload traffic to the small cells, the UE needs to periodically perform inter-frequency scanning, which causes service interruption between the small cell base station and the UE, as well as causing relatively large energy consumption at the UE. In fact, it has been shown in TR 36.839 that significant power consumption can occur in a UE when inter-frequency measurement is continuously performed over the cell-specific reference signals, while a relatively low impact on offloading potential is observed.
Small cell discovery in a macro deployment with small cells has been considered in 3GPP TR 36.839, especially macro-assisted small cell discovery. In one of the methods, a dedicated discovery reference signal (DRS) is used to trigger the detection of the small cells. In another method, DRX/DTX was used to perform inter-frequency measurements for small cell discovery.
A proximity-based small cell discovery process is illustrated in FIG. 1A. FIG. 1A shows the message exchange between a UE 301 and eNB 302.
In FIG. 1A, detection of the proximity of small cells 312 is usually performed by using a received signal strength (RSS) based radio map. The entries of the map, called radio fingerprint, are used to ascertain the proximity of the UE to the small cell. An illustration of an exemplary fingerprint database structure is shown in, FIG. 1B. Such a database may be created by using past experience of the terminals and may be stored either at the macro cell base station or at the terminal. Proximity of small cell is detected 312 when the RSRP measured at the UE is at a given range of the RSRP in the database (the so-called ‘fingerprint match’).
After proximity is detected, the UE either attaches to that small cell directly, or it performs some measurement of the small cells that are identified as ‘proximity cells’. One example of such measurement 321 is given in FIG. 1, which includes the measurement of, for example, RSRP, RSRQ, of the small cells, and is measured over the carrier frequency of the small cell. Such measurements are then reported back to the macro base station, and offloading to the small cell occurs when a given criterion is satisfied, for example, when the measured signal strength is higher than a threshold.
Generally, small cell discovery/selection focus on providing efficient measurement configuration (e.g., measurement gap or time duration, rate of measurement etc) for the purpose of power saving. One of the problems that has generally been overlooked is that a good backhaul connection from the small cells to the core network is not always guaranteed. Unlike macrocells, small cells are typically located in hard-to-reach near ground locations, rather than in the clear, high above rooftops, and so, small-cell backhaul can take different forms, using various wireless or wired technologies. In particular, unlike macro base stations that are connected to the core network via fibre-optic connections, where the quality of the connection is almost always guaranteed, the small cells may connect to the core network wirelessly, with a possibly uncertain quality of connection to the core network.
It is therefore possible that a UE can detect strong received signals from a small cell base station (indicated by the RSRP/RSRQ measurement), but cannot effectively communicate with the core network due to an interrupted backhaul connection between the small cell and the core network. As a result, a UE only using the received signal power as measurement for small cell discovery may suffer from severe performance degradation even when a strong link between the small cell base station and the UE exists, which causes delays, interruptions of communications, and unpleasant customer experience and is therefore highly undesirable. Such is illustrated in FIG. 2, as FIG. 2 illustrates a representation of a macro and small-cell deployment. FIG. 2 depicts a core network 350 connected to a macro cell base station 352 via a fiber connection 351, which offers a good quality of connection. It also shows a UE 353 which is connected to a small cell 354, which is connected wirelessly via wireless connection 355 to the core network 350. The wireless connection 355 offers an uncertain level of connection quality and can vary.